The following example shows how to compute an SA-refined omit map (Hodel, Kim, and Brünger 1992). The omitted region consists of an 8 Å sphere around residue 20. Other selections can be specified here. The immediately surrounding atoms, consisting of a 3 Å shell, are restrained to avoid artificial movement into the omitted region. The slow-cooling protocol is used for simulated annealing (see Section 13.1.3).